Flash memory employs floating gate transistors in a plurality of memory cells. One or more charge levels may be programmed onto the floating gates of the floating gate transistors in order to store data. The threshold voltage level of a floating gate transistor depends on the level of charge programmed therein. Thus, the data stored within flash memory is read by comparing the threshold voltages of the floating gate transistors to one or more reference voltages. This mechanism is used in both single-bit and in multi-bit flash memories.
A floating-gate transistor is capable of storing a voltage on its “floating gate” for a period, typically many years. The threshold voltage of the floating-gate transistor will change depending on a value currently stored on the floating gate. In a single-bit flash cell, for example, the threshold voltage of the cell will be one of two voltages depending on the level of charge stored in the floating gate transistor. Thus, in some implementations of flash memory, the stored value is read by comparing the threshold voltage to a reference voltage. If the threshold voltage is less than the reference voltage, then the output is considered to be a first binary number (such as a 1). If the threshold voltage is greater than the reference voltage, the output is considered to be a second binary value (such as a 0). In a sense, a floating gate transistor has multiple threshold voltages, each exhibited only when a certain voltage is stored on the floating gate.
The levels of these multiple threshold voltages of a floating gate transistor will change over time due to physical changes after repeated erase and program cycles, or for other reasons. A group of flash memory cells will therefore exhibit a change in their overall threshold voltages distribution over time. And if the threshold voltages change as a memory device ages, then it can be useful to adjust the reference voltages used to read the memory cells as the device gets older. This can, for example, result in lower error rates. If the reference voltages are not varied, the device may exhibit higher error rates as it ages.
The description in this section is related art, and does not necessarily include information disclosed under 37 C.F.R. 1.97 and 37 C.F.R. 1.98. Unless specifically denoted as prior art, it is not admitted that any of the described related art is prior art.